the interventional pas de trois is from the original Taglioni scheme so far as i can tell. it is a magical and mysterious in its way. the last act of LA BAYADERE originally included a similar pas de trois (or pas de deux a trois) when Nikiya comes phantomlike between Gamzatti and Solor in the last act as a Shade only Solor can see. the 'reconstruction' of this trio by Vikharev in his production of the ballet was one of its more memorable highlights: it's all shown in full light and yet one suspends disbelief and understands that only Solor can see this white-clad shadwo, if mem. serves in this act Nikya's Shade is costumed with the sleeve/veils of the ensemble Shades from the Kingdom of the Shades scene.
The Shade pas à trois in the Paris "La Sylphide" wasn't originally there. Yet it was interpolated some ten years after the premiere by Taglioni from his own 3-Act ballet "L'Ombre" created for St. Petersburg.
Nikiya does indeed appear in her Shade costume during the trio with Gamzatti and Solor in the Vikharev reconstruction.
Nanarina, of course the current Paris Sylphide by Lacotte involved a great deal of research, but choreographically it is still in essence a pastiche from the 1970's, while interpretation-wise it doesn't carry the same weight of tradition as the Danish version.